The present invention relates to an exterior panel structure of a motor vehicle and, more particularly, to a body structure of a motor vehicle having exterior panels which are made of synthetic resins.
In the art of motor vehicles, there is an increasing demand for a light-weight body structure which cuts down fuel consumption. One of expedients heretofore proposed to reduce the weight of a vehicle body is forming various structural parts out of synthetic resins. Incidentally, the use of synthetic resins for producing vehicle parts eliminates the need for measures against corrosion.
However, the problem with synthetic resins is that due to a coefficient of thermal expansion which is greater than that of steel it undergoes significant expansion and contraction depending upon the temperature, affecting designed dimensions of products. For this reason, the application of synthetic resins in a vehicle body has customarily been limited to those parts located inside a vehicle cabin which are small in area and subjected to relatively small temperature variations, a covering of a bumper which may be arranged with some clearance defined between itself and a vehicle body, etc.
To further promote a light-weight design of a vehicle body, however, it is desirable that the application of synthetic resins be extended even to fender panels, door panels and other large-size exterior panels. It is especially desirable from anticorrosion standpoint that those panels at the sides of a vehicle body which easily rust due to chipping and other causes be made of synthetic resins.
Exterior panels of a motor vehicle need to be surely mounted so as not to come off or warp when subjected to vibrations of a vehicle body, temperature variations, etc. To meet this demand, exterior panels are usually fixed along all of their edges to rigid members which are adapted to ensure mechanical strength of the vehicle body. Even exterior panels made of synthetic resins, therefore, may be fixed to rigid members along all of their edges.
However, exterior panels of a motor vehicle are significantly susceptive to variations of the ambient temperature. Where exterior panels are made of synthetic resins and dimensioned to serve as, for example, door outer panels of a two-door vehicle, the dimension of each panel in the longitudinal direction of the vehicle is changed by about 6 millimeters from the most expanded condition to the most contracted condition. Moreover, because each exterior panel has a substantial area, the expansion and contraction occurs not in one direction only but two-dimensionally.
Meanwhile, rigid members to which external panels are mounted are generally made of steel or fiber-reinforced plastics. Such materials, compared to synthetic resins usable for exterior panels, shows a small coefficient of linear expansion to heat.
Where an exterior panel made of synthetic resin is fixed along all of its edges to a rigid member which is made of a different material as discussed above, it undesirably undulates when the ambient temperature is high and cracks along fixed portions thereof when the temperature is low.
It is therefore a prerequisite that exterior panels made of synthetic resins be mounted to a vehicle body in a special manner which accommodates changes in the dimensions of the panels. One approach to meet this prerequisite is allowing a panel mounted on a rigid member to slide in a plane thereof and, yet, preventing it from coming off the rigid member. In such a case, an arrangement is made such that expansion and contraction of the panel are accommodated by edge portions of the panel.
A problematic situation which arises here is that edge portions of an exterior panel are usually joined to those of other exterior panels, lamps, a hood and others adjacent thereto. An attempt to arrange an exterior panel made of synthetic resins in the same plane as nearby exterior panels as has been the case with traditional steel panels results in the need for a clearance between the neighboring panels which is large enough to accommodate changes in the dimensions of the resin panel, so the resin panel may be prevented from being abutted against and damaged by the other panels when expanded most. Especially, for a door outer panel or like exterior panel associated with an openable member, it is necessary that a clearance of about 5 millimeters be constantly defined between the openable edge of the panel, i.e., rear edge in the case of a front-hinge type door and the front edge of a rear fender panel or the like which neighbors the former, taking account of scattering of the whole vehicle body configuration, errors particular to door hinge mounting steps, errors in door lock mechanisms, warps due to aging, etc. Therefore, if an arrangement is made in such a manner as to accommodate expansion of the door outer panel made of synthetic resin at the portion in question, the clearance will become as great as about 11 millimeters when the panel is contracted in cold weather. It follows that where a rear fender panel, too, is made of synthetic resin, an extra clearance needs to be defined between the door outer panel and the rear fender panel in order to accommodate variations in the dimensions of the rear fender panel, resulting in a considerable clearance in total.
Such a clearance between nearby exterior panels would critically degrade the appearance of a motor vehicle while allowing externally derived noise to enter the vehicle cabin.